To produce a negative ion source, it is necessary to first create and confine a plasma which produces positive ions, electrons, excited neutral molecules, and some negative ions. The confinement is obtained by various magnetic field configurations, usually produced by permanent magnets. The conditions in these self sustained plasmas are not optimum for producing negative ions, and their ion concentration is generally low. It is also very difficult to extract these negative ions because of the potential barrier that exists across the plasma sheath at the extraction aperture. This potential is proportional to the electron temperature in the plasma. Note here that this potential tends to pull the positive ions out of the plasma. Thus, a positive ion source is somewhat easier to build. In such a source, the positive ions just fall down the potential well and out of the plasma.
To circumvent this problem in a negative ion source, an additional magnetic field, called the filter field, is used to screen the plasma, and in particular its high energy electrons, from the region near the extraction aperture. In this manner, the negative ion source is magnetically broken into two chambers. In the first chamber, a discharge creates a plasma with a relatively high electron temperature which is necessary to sustain the discharge and to generate the vibrationally excited molecules necessary for low temperature H.sup.- production. In the second chamber, near the extraction aperture, a plasma with a very low electron temperature is produced. Even though the filter field prevents the high energy electrons from entering the second chamber, the low energy electrons cross the filter field. Some low energy electrons enter the second chamber as a result of negative ions produced in the first chamber, near the filter, crossing the filter field and then becoming neutralized in stripping collisions. This process helps to produce the low temperature (kT.sub.e .ltoreq.1 eV) plasma found there. Another possibility is that those low energy electrons, whose relative velocity with respect to the positive ions in the plasma is low, form a quasi-bound particle that appears as a neutral entity while it crosses the magnetic field, but disassociates when it enters the second chamber, thus also helping to create the low temperature plasma in this region. In any event, the potential barrier across the plasma sheath at the extractor is now quite low due to the low electron temperature, and negative ions with energies of the order of 1 eV can escape and be extracted. Low temperature negative ions are necessary if the high brightness beams required for some applications are to be formed.
Negative ion sources have found useful applications in plasma fusion devices for diagnostic, neutral beam heating, and current drive where very high currents are required; and in high energy accelerators where low emittance beams are often necessary. More recent high-energy accelerator applications have been in the area of neutral particle beam weapon (NPB) systems. In this application, ion beams are produced, accelerated to high energies, directed towards a distant target; and are then neutralized by stripping the excess lightly bound electron. This produces a neutral particle beam which propagates to the target without being affected by the earth's magnetic field.
In order for the neutral particle beam to be of a reasonable size (about the size of the target) when it reaches the distant target, a very low emittance beam is required. Also, if the target is to be damaged, a reasonably high current is needed. These two requirements, when taken together, mean that a very high brightness beam is necessary. In this application, it is also necessary for the system to operate autonomously. Thus, a well behaved automatically controlled source is required. The ability to automatically control the source is also useful in many of the other applications.
This need for low emittance, high brightness beams places severe requirements on the accelerator, magnetic optics, neutralizer, and, in particular, on the ion source. If current is not lost in accelerating the beam, then the brightness and emittance can be no better than that produced by the ion source and, generally, the brightness will be decreased each time the beam is manipulated in any way. The emittance, .epsilon., at the source is given by ##EQU1## where r is the radius of the extraction aperture, k is Boltzmann's constant, K.E. is the kinetic energy received at extraction, and T.sub.i is the temperature of the negative ions just after extraction and acceleration to ground potential. This equation is just an expression of the Heisenberg uncertainty principle, since (T.sub.i ) is proportional to the random momentum of the negative ions in the beam and r is proportional to the uncertainty in position of the ions. The brightness is given by ##EQU2## which is proportional to the ratio of the current density, J, given by ##EQU3## to the ion temperature, T.sub.i .
Efforts have been made to develop two types of negative ion sources for NPB weapons. Surface plasma sources have been developed which produce very high brightness beams but only for very short pulses and low duty factors, and the multichamber or so called "bucket" type volume sources which operate continuously (CW), but have not yet produced beams of the desired brightness. It is unlikely that either of these sources, in their conventional configurations, will meet the brightness and duty factors required for an advanced NPB weapon system.
To produce a negative ion source that meets or exceeds these desired brightness levels, it was felt that a somewhat radical departure from the current approaches was necessary. Therefore, Roberts, Lavan and Strickland disclosed an electron beam driven negative ion source (U.S. Pat. No. 5,391,962, issued Feb. 21, 1995) which overcomes the limitation these conventional configurations place on themselves.
In any negative ion source, there are mechanism (or reactions) that produce the negative ions and processes that destroy them. The number of negative ions per unit volume is the result of the balance between these constructive and destructive processes. At the extraction aperture, where the useful beam is generated, the extraction itself becomes one of the destructive or loss mechanisms. Thus, the only negative ions that can be extracted to form a negative ion beam are those that are born within one mean free path, on average, from the extraction aperture. This mean free path is determined primarily by the most dominant mechanism that destroys the negative ions. The optimum ion source will therefore consist of independent mechanisms for maximizing the desired conditions near the extraction aperture. The negative ion production process that yields the lowest temperature ions in a cw source must be maximized, while, at the same time, independently minimizing the conditions for the process that most quickly destroys them. The ion sources discussed above do not allow for this independent adjustment of optimum conditions. Again, these conditions need to be produced and maintained only near the extraction aperture.
The lowest temperature negative ions are produced by dissociative attachment of low energy electrons to hydrogen molecules in high vibrational states; EQU e.sub.i.sup.- +H.sub.2 *.fwdarw.H.sub.2.sup.- .fwdarw.H+H.sup.-
where e.sub.i .sup.- is a low energy electron; H.sub.2 * is a vibrationally excited hydrogen molecule, H.sup.- is a negative hydrogen ion and H is a neutral hydrogen atom. (Hydrogen is used here as an example. The vibrationally excited molecule could just as well have been deuterium or tritium. Any other particle that has an affinity for forming negative ions, such as Li, Cl, O, etc., might, at least in principle, also be used.)
The two dominant destruction processes are collisional detachment caused by hot, i.e. fast, electrons: EQU e.sub.f.sup.- +H.sup.- .fwdarw.2 e.sup.- +H ,
where e.sub.f .sup.- is a fast electron; and associative detachment or recombination which is represented by: EQU H+H.sup.- .fwdarw.H.sub.2 +e.sup.-.
The equilibrium negative ion concentration is established by a balance between these constructive and destructive processes. However, in some sources, especially when low work function materials like cesium are used, collisions with the walls near the extraction aperture do play a part in the kinetics that produce the negative ions. Another loss mechanism important during extraction, while the H.sup.- ions are being accelerated, is collisional stripping with background hydrogen gas: EQU H.sup.- +H.sub.2 .fwdarw.H+H.sub.2 +e.sub.i.sup.-.
It is this reaction that limits the H.sub.2 gas pressure that can be used in the sources.
In conventional multichamber or "bucket" type negative ion sources, a low voltage (.about.100 Volts), high current (several hundred amps) dc discharge is used to produce the vibrationally excited H.sub.2 molecules. This type of discharge in the first chamber not only produces the desired vibrationally excited H.sub.2 molecules, but it also produces many other species such as H, H.sup.+, H.sub.2 .sup.+, and H.sub.3 .sup.+ which cross the magnetic filter and contribute to the reactions which occur in the extraction region in an undesirable way. Another problem with these discharges is that they always suffer from erosion of the cathodes and filaments that are used to start the discharges. Long filament life is difficult to obtain; therefore, high frequency discharges, which are free of this defect, have also been used. The frequencies used generally range from several hundreds of kilohertz to tens of gigahertz. In this case, the plasma containment vessel can be made from insulators or metals. When metals are used, the vessel can be made in the form of a resonator. In either case, magnetic fields are used to confine the discharge. But, since all of the energy required to produce and maintain the plasma must be supplied by the high frequency power source, the equipment is much more complicated, generally larger, and always more expensive.
The surface plasma sources utilizes a low voltage, high current arc confined to a region near a ceseated converter surface by a strong magnetic field to produce both H.sub.2 vibrationally excited molecules and negative ions. In some of these sources, the negative ions produced on the ceseated surface are projected towards the extraction aperture by an electric field applied at the converter surface. In other sources, the negative ions produced on the ceseated cathode are projected towards the low magnetic field region, but not towards the extraction aperture. In both of these sources, the surfaces near the extraction aperture becomes ceseated and additional negative ions are produced there. This makes these sources more complicated and even more difficult to understand than the multichamber cw sources, even when cesium is used in the cw sources.
In all of the above cases, the discharges are self-sustained. This means that the ratio of the electric field (E) to the number of particles per cubic centimeter (n) cannot be optimized simultaneously for both the desired discharge operation and the production of H.sup.- ions. The operation of the arc or discharge is critically dependent on this ratio (E/n). If E/n is too low, the discharge cannot be maintained, and near the threshold value of E/n, for self-sustained operation, the discharge tends to be unstable and noisy. The population of the H.sub.2 molecules in the high vibrationally exited states is even more sensitive to E/n. The range of value of E/n for which the arc operates satisfactorily is far from the optimum value for the excitation of H.sub.2 to high vibrational states by collisions with the electrons in the discharge. Therefore, in these sources, the best operating conditions are established by a trade-off between two conflicting requirements on the arc voltage and the H.sub.2 pressure or flow rate. An ideal source would allow for these two requirements on E/n to be optimized independently. Of course, in this case, the discharge cannot be self-sustained and it must be sustained by some other means.
This fact was recognized by Roberts, et al, when they felt that a radical departure from the above approaches was necessary. Therefore, they disclosed "An Electron Beam Driven Negative Ion Source", (U.S. Pat. No. 5,391,692, cited hereinabove), wherein an electron beam produced in one chamber is used to sustain in another chamber a discharge whose conditions are independently adjusted for optimum production of vibrationally excited hydrogen molecules and in yet another chamber there is an independently controlled source of low energy electrons in a region near an extraction aperture. In this manner, the optimum conditions for creation of H.sup.- ions by dissociative attachment are produced so that the current and brightness of the H.sup.- beam may be maximized. However, this device loses sight of the lesson learned from the previous efforts, and does not lend itself to utilization of the many nuances that have been developed. The Roberts, et.al. device uses a high voltage electron gun and a fragile thin foil. It does not make use of magnets to confine the discharge plasma. It is large, very expensive, and unnecessarily complex , and it uses relatively large amounts of H.sub.2 gas, which, along with the high voltages, can cause safety problems that either limit its usefulness or cause additional expensive complexities, and it does not provide for automatic operation.
Therefore, it is an object of this invention to provide an automatically controlled negative ion source which uses as much of the current state-of-the-art technology as possible, but yet allows independent control of E/n for the optimum production of vibrationally excited molecules. Thus, in accordance with this invention, conditions are created near the extraction aperture for the extraction of maximum low temperature H.sup.- ions and the production of very bright negative ion beams. To accomplish this, a direct current (dc) discharge that does not run self-sustained is required.